A solid oxide fuel cell (SOFC) is known as a versatile and highly efficient fuel cell. Since the SOFC has a high operation temperature in order to increase ion conductivity, it is possible to use air discharged from a compressor of a gas turbine as air supplied to a cathode (oxidant). Also, it is possible to use high-temperature fuel and exhausted heat which are unused in SOFC, as a fuel and an oxidant in a combustor of the gas turbine. Besides the SOFC, a molten carbonate fuel cell is known as a fuel cell having a high operation temperature. Similar to the SOFC, use of the exhausted heat in link with the gas turbine is considered.
Therefore, for example, as described in the following Patent Literature 1, as a power generation system capable of achieving high efficiency power generation, there have been proposed various types combining the SOFC, the gas turbine, and the steam turbine. The combined system described in Patent Literature 1 is provided with the SOFC and the gas turbine having a gas turbine combustor, which combusts exhaust fuel gas and exhaust air discharged from this SOFC, and a compressor, which compresses air and supplies it to the SOFC. As a system with which maximum power generation efficiency can be obtained, there is the system described in Patent Literature 2. With this system according to Patent Literature 2, it is described that, by decreasing a utilization rate of a raw fuel gas of the solid oxide fuel cell to 65 to 80%, a hydrogen concentration in the vicinity of an exit portion of the solid oxide fuel cell is increased, and power generation performances are improved.